Batteries power cellular telephones and other portable or mobile electronic devices, and these batteries need to be recharged frequently. To reduce the need for recharging the batteries in the mobile electronic devices, the devices may be turned-off, or the devices may remain on while operating in a battery-save mode. However, the battery-save mode of many cellular telephones still has a current drain as high as two to three milliAmperes (mA) because all of the clock generation circuits in the devices continue to operate during the battery-save mode. Thus, the battery-save mode of these cellular telephones still shortens the lifetime of the batteries by a significant amount.
To increase the lifetime of the batteries, other cellular telephones remove the primary supply voltage from a majority of the circuits inside the cellular telephones during the battery-save mode. However, these cellular telephones cannot receive incoming telephone calls when operating in the battery-save mode. Therefore, to avoid missing incoming calls, these cellular telephones must periodically cycle out of the battery-save mode and into an active mode to re-apply the primary supply voltage to the circuits inside the cellular telephone. This cycle may be repeated several times per second. The high rate of on and off switching of the primary power supply requires low-value by-pass capacitors to be connected to the power supply lines of the primary power supply. However, the low-value by-pass capacitors provide very limited protection from power supply transients and require specialized charging and discharging circuits to quickly remove and re-apply the primary power supply voltages. Another disadvantage to switching off the primary supply voltage during the battery save mode is the loss of programmed control data that must be reprogrammed when the primary supply voltage is re-applied in the active mode.
Accordingly, a need exists for a device having battery-save circuitry and a method of operation that extends battery life without turning off the primary power supply so that specialized charging and discharging circuits are not required, programmed data can be retained, and high value by-pass capacitors can be used in the device.